The art of theatrical illumination has been greatly advanced in the recent past. In addition to well-known wash (or general area) lights and spot lights, recent systems have been devised in which many parameters of a light beam projected by a luminaire can be varied by remote control, including the orientation of the luminaire with respect to pan and tilt, and the diameter, shape, divergence, color and intensity of the beam. Bornhorst U.S. Pat. No. 4,392,187 describes a system which has been found to work well in actual practice. A specialized type of theatrical luminaire capable of projecting an image on a stage screen or backdrop is described in Bornhorst U.S. Pat. No. 4,779,176. While standard wash and flood lights are typically provided with parabolic or spherical reflectors, projection type luminaires are increasingly being provided with ellipsoidal reflectors. In all the discussed types of modern theatrical luminaires, and especially with respect to automated luminaires with pan and tilt heads, a consistent design goal has been to decrease luminaire size and weight while increasing the number of functions which can be performed and the mobility of the lighting instrument itself.
An important parameter for all luminaires is light intensity. While acceptable known systems have used electronic means of varying light beam intensity, mechanical means of varying intensity (or dimming) are often preferable. At least three major types of mechanical dimming apparatus are known: irises, shutters and blades. Unfortunately, none of the mechanical dimmer systems is entirely acceptable.
Iris dimmers are typically composed of a plurality of curved, opaque elements, or leaves, carried in two concentric and intermeshing rings. The iris is operable to vary the diameter of an aperture formed by the inner edges of the leaves. As the rings rotate, the leaves pivot about pins which secure each end of each leaf to one or the other of the two rings, such that each leaf is secured to one ring at one end and to the other ring at the other end. As the diameter of the aperture decreases, the amount of light which can be projected through the iris is diminished. Brenkert U.S. Pat. No. 1,591,211 describes a typical iris dimmer in a theatrical luminaire.
However, several difficulties arise from use of an iris dimmer. The iris must not be located at or near a focal point of an optical system, for example in the projection gate of an ellipsoidal spot light projector, or the inner edges of the iris leaves will be projected as an image. Even when located away from a focal point, operation of the iris affects the depth-of-focus of the optical system. As the iris aperture is made smaller, the depth-of-focus increases so that an image formed in the aforementioned projection gate is brought into focus even if prior adjustment of a projection lens included in the optical system has made the image out-of-focus.
Moreover, iris dimmers are complex in terms of the number of moving parts, the tolerances required for the dimmer to function and the actuator or other driving linkages. In addition because an iris dimmer typically may not be fully closed, additional equipment to fully block the light beam is often required.
Another class of dimming apparatus known in the prior art is shutter dimmers. Shutter dimmers are typically composed of a plurality of generally straight, opaque elements arranged either in side-by-side fashion transversely across a light beam path or in a radial fashion extending from a hub in the center of a light beam path to a point on the periphery of the path. The shutter is operable to vary the spacing between the edges of the shutter elements. Each element pivots in coordination with all other elements to obstruct the projection of light rays by an optical system and thereby control the intensity of a projected beam. Steel U.S. Pat. No. 3,333,094 depicts the general characteristics of shutter dimmers.
Shutter type dimmers offer means to avoid the design difficulties encountered with iris dimmers. Shutter dimmers are simpler to construct than iris dimmers, and induce no effect upon the depth of focus of the optical system. Shutter dimmers are frequently used in optical systems having parabolic or spherical dish reflectors, for example floodlights and searchlights, which project no image. In systems capable of projecting an image, shutters may be disadvantageous because the shutter elements tend to redirect portions of a light beam incident thereupon as the elements move through intermediate positions between fully open and fully closed. This result may be effectively countered by: 1) using non-specular elements, for example elements painted a non-reflective black; 2) using smaller elements, although more such elements are required; 3) employing a radial arrangement of shutter elements such that portions of the beam are reflected in different directions; 4) providing baffles around the shutter or even enclosing the shutter within a housing containing a lamp and reflector; or any combination of the aforementioned techniques.
However, the known methods by which the disadvantages of shutter dimmers may be overcome are themselves disadvantageous when attempting to construct a modem motorized luminaire in which small size, light weight and quick maneuverability using the smallest possible motors are the primary design criteria. For example, extra baffles and housings increase both the size and weight of the luminaire. In addition, increasing the complexity of the shutter increases cost and construction difficulties so as to negate the desired advantages over an iris dimmer design.
A third type of dimmer known to those of skill in the art is a blade dimmer. A blade dimmer is composed of an opaque element, or blade, which is introduced into a light beam path to reduce the intensity of the projected beam. In operation, the blade may enter the beam path via motion along a track or guide, or through arcuate motion about a pivot point adjacent to the light beam path. Although dual-blade dimmers are known, generally a blade dimmer has only one moving part consisting of the blade coupled to an actuator, as contrasted with the plurality of elements commonly required for an iris dimmer or a shutter dimmer. The blade may have a straight leading edge which first enters the light beam path, but such a construction disadvantageously tends to dim one side of the projected beam more than the other as the blade is rotated into the beam path. For this reason, blade dimmers often feature serrated, ragged, or severely spiked leading edges, so that the dimming effect is less noticeably asymmetrical.
Blade dimmers are capable of providing several advantages over iris and shutter dimmers. A blade dimmer is significantly simpler to construct than a shutter dimmer, having only the one moving part. Full-field dimming is more difficult to accomplish, however, requiring a complex shape of the leading edge of the blade. The simplicity of the resulting mechanism offers sufficient utility to offset the difficulty of fabricating the blade. And, like the shutter dimmer, the blade dimmer presents no problems affecting the depth of focus of an optical system.
Nonetheless, prior art blade dimmers leave much to be desired. For example, in a typical spot light projector for entertainment lighting, having an arc-lamp light source optically coupled to an ellipsoidal reflector and one or more projection lenses, a blade dimmer must be reflective to avoid deterioration due to heat absorption. The arc lamp of such a system is positioned at a first focal point within the reflector such that light rays emanating from the lamp will converge upon a second focal point at which a projection gate is located. Depending upon the placement of the blade, the reflected light rays tend to converge upon a third focal point where a partial image of the arc light source is formed. When the blade fully intercepts the beam, all of the reflected light rays will converge upon the third focal point. This third focal point may occur anywhere along the beam path between the blade and the reflector, at the periphery of the transparent envelope surrounding the arc light source, or even at a front seal of the envelope, where excessive overheating and subsequent premature failure of the lamp may result. This unsatisfactory result is even more serious in the temperature and space constrained environment of a modern motorized luminaire with a pan and tilt head. In such a device, axial beam path space and weight are necessarily severely constrained.
A blade dimmer may be tilted with respect to the beam so that reflected light is not returned to the light source but is reflected into a side housing. However, sufficient angling of the blade to achieve the desired result requires much more axial beam path for the dimmer assembly and so is an undesirable solution in a compact, lightweight luminaire.
Although there are several types of mechanical dimmers known to the art, none is fully acceptable for use in a modern theatrical luminaire. A need exists for a new mechanical dimmer which provides a simpler construction and is lighter and smaller than the known mechanical dimmers. An acceptable mechanical dimmer must use the least possible axial beam path space and be able to be rotated quickly to provide a continuous range of dimming from slight through complete reduction of the intensity of the projected beam.
It is an object of the present invention to provide a blade dimmer which avoids destructive reflection of the blocked light beam.
It is another object of this invention to provide a blade dimmer which is adapted for use in a high heat environment modern motorized pan and tilt luminaire.
It is still another object of the present invention to provide a blade dimmer adapted to a modern theatrical luminaire which diffuses the reflected beam with minimal induced vibration due to imbalance of the blade when in operation.